I did not have anything specific in mind. It just seemed, based on the symptoms, that FastMail was messing up the headers, and I thought resetting them might be a circumvention allowing the message to be sent.

Of course, the issue needs to be fixed, but at least this provides additional evidence of what is going on, as well as a simple way of getting around the problem should it rear its ugly head.

I think it's reasonable to just wait for the code that's on the beta server that fixes the problem to go live. I think that's what the developers have decided, and I'd make the same choice. A code migration specifically to fix this isn't worth it. Better spending the time on other things. We have 2 good workarounds. Bummer for folks who get an uninformative error message and don't discover a workaround.